Mosses Influence Vascular Plants In Rich Fens Through Effects On The Biogeochemistry Of Shallow Soils

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Surprisingly little is known about the effects of non-Sphagnum mosses on soil processes or associated vascular plants, despite known strong effects of Sphagnum on the biogeochemistry of acidic peatlands. Mineral-rich fens, which are groundwaterfed peatlands where phosphorus often limits plant growth, frequently have a dominant layer of "brown" mosses covering the peat surface and an unusually high diversity of forb species rooted in shallow soils. I hypothesized that rich fen mosses would have biogeochemical effects that in turn would facilitate forb species both below- and aboveground, thereby influencing plant species diversity overall. To investigate the effects of rich fen mosses on biogeochemical processes, I performed a moss removal experiment at three New York fens. Where mosses occurred, redox potential was greater, reduced iron concentrations were lower, and phosphorus uptake by plants was higher than where mosses were removed. More oxidized soil conditions underlying mosses may have stimulated (1) greater phosphate supply through increased microbial mineralization; (2) greater phosphorus demand via microbial uptake, plant uptake, and iron-phosphorus reactions; and (3) greater phosphorus acquisition through improved plant vigor and mycorrhizal colonization -- thus improving forb phosphorus uptake in a phosphorus-limited environment. Below- and aboveground responses of vascular plants to mosses were examined in the moss removal experiment and in a survey of moss-covered and naturally moss-free areas. Total root biomass and root length density from shallow soil core sections were greater with mosses present than naturally absent. For two forb species tested experimentally, root length, surface area, and branching were 1.5-2 times greater where mosses occurred. Forb seedling survival, numbers of individuals, percent cover, and species density all were greater where mosses occurred; in contrast, graminoids did not respond. Patterns of root morphology and species establishment were consistent with demonstrated biogeochemical effects of mosses on redox potential and phosphorus cycling in shallow soils. This work emphasizes the role of positive plant species interactions in rich fens, suggesting that mosses may help diverse, shallowly rooted forb species to succeed under saturated and phosphorus-limited conditions. The presence of mosses thus may have key implications for the plant species composition and diversity of these unique wetlands.
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